Systemic onset juvenile idiopathic arthritis is a rare form of JIA (juvenile idiopathic arthritis) with an incidence of 0.4–0.8 per 100,000 . Juvenile idiopathic arthritis (JIA), as defined by the International League of Associations of Rheumatologists, comprises various childhood arthritides of unknown cause in children less than 16 years of age and lasting for at least 6 weeks. Systemic onset JIA is a disease characterised by repetitive disease flares, with remissions in the interim.
The usual age at presentation is between 1–6 years with equal sex incidence. Systemic symptoms like fever, rash and lymphadenopathy could precede the development of arthritis. Other extra-articular manifestations  include anaemia, and inflammation of the pleura, pericardium or the peritoneum. One quarter of the cases develop severe destructive arthritis with time. The rash is typically transient, blanching macular or maculopapular; it occurs during the fever peaks.
It is classified as a subtype of juvenile idiopathic arthritis, with no sex bias or peak age at onset. Extraarticular features include daily spiking fevers, fleeting salmon-coloured macular rash, lymphadenopathy, hepatosplenomegaly and polyserositis. Persistent disease leads to severe growth impairment, in addition to the damage caused by steroid therapy. The main concern is the precipitation of macrophage activation syndrome (MAS)  by triggers like viral infections, drugs and external stresses. A systemic onset juvenile idiopathic arthritis is unique among childhood arthritides due to the association with the macrophage activation syndrome (MAS).
Both an infectious-pathogen-triggered and an auto-inflammatory aetiology have been proposed for SOIJA. Even though the disease triggers are unknown, SOJIA is driven by innate pro-inflammatory cytokines, perpetuated by synergy with endogenous ligands like S-100 proteins. The most important cytokine involved is interleukin-1 (IL1). Other cytokines involved include IL-6, M-CSF (macrophage colony-stimulating factor), tumour necrosis factor (TNF) and IL-18. Interleukin-1 has pleiotropic effects, with upregulation of its own transcription and other cytokines, stimulation of cartilage and bone destruction, as well as elevation of plasma proteins during SOJIA flare. Treatment with IL-1 and IL-6 inhibitors has been shown to be highly effective. Another pathogenetic explanation of SOJIA involves the alternative activation of monocytes and macrophages, with probable deficiencies of IL-10 and regulatory T cells.
Mortality in SOJIA is related to complications of systemic inflammation (e.g., MAS and amyloidosis) and those due to immunosuppressive medications. In addition, SOJIA is complicated by limitations in functional outcome due to arthritis and long-term damage due to chronic inflammation.
Nearly 10% of patients with active SOJIA are diagnosed with MAS, an acquired form of haemophagocytic lymphohistiocytosis (HLH). It is characterised by well-differentiated macrophages with haemophagocytic activity. It is a life-threatening condition presenting with persistent fever, pancytopenias, liver abnormalities, coagulopathy and central nervous system dysfunction. MAS resembles multi-organ dysfunction and shock. Laboratory features consist of liver function derangement (hepatobiliary dysfunction), abnormal coagulation profile, pancytopenia, hyperferritinaemia, and elevation of acute phase reactants.